Metallic alloys are called monotectic alloys that form at higher temperatures two immiscible liquid metallic phases. Upon cooling below the monotectic temperature, at first one of the liquid phases, then below the eutectic temperature, the other liquid phase is solidified and in this way a solid monotectic alloy is obtained. Usually two macroscopic layers are formed on each other, due to the density difference between the two liquid phases.
Monotectic alloys such as Al—Pb, Cu—Pb, Al—Bi, Al—In and others are applied in different technological fields, such as bearing alloys or high-temperature super-conductors. Monotectic alloys would offer their best performance if one of the phases would be dispersed as small droplets in a homogeneous way in the other phase (called matrix). The smaller is the size and the more homogeneous is the distribution of the dispersed phase, the better are the expected properties of monotectic alloys. However, this requirement is opposed by the presence of the interfacial energy between the phases, making the droplets coalesce and by the density difference between the two liquid layers, making the layers vertically separate (sediment) with a velocity being higher for a higher droplet size. These effects are enhanced by the effect of the interfacial gradient force (Marangoni force), generally pulling the droplets towards places with higher temperatures, if there is any temperature gradient in the system. Nevertheless, freezing liquid alloys without a temperature gradient is impossible, thus the latter effect also acts against the homogeneous distribution of droplets [J. Z. Zhao, S. Drees and L. Ratke: Strip casting of Al—Pb alloys—a numerical analysis, Mater. Sci. and Eng., A282, 262-290 (2000); G. Kaptay: On the temperature gradient induced interfacial gradient force, acting on precipitated liquid droplets in monotectic liquid alloys, Materials Science Forum, 508, 269-274 (2006)].
Due to the above circumstances, the key for producing monotectic alloys is the stabilization of dispersed droplets to prevent their coalescence and sedimentation. The ways known in the literature to produce monotectic alloys with homogeneous distribution of the second phase are summarized below.
Fast Cooling and Freezing
If a system of two immiscible liquid alloys is mixed at a high rotational speed with a special mixer, a system consisting of the dispersed droplets can be formed. If this system is quickly frozen, the dispersed droplets are frozen and in this way a monotectic alloy with homogeneous distribution can be obtained. Using this technology an ideally homogeneous distribution of the droplets can never be achieved, but this ideal situation can be approached by increasing the speed of mixing and freezing.
Such a technology is described by the following literature sources for the Al—Pb system: T. Ikeda, S. Nishi and T. Yagi: Manufacture of homogeneous ingots of Al—Pb alloy by casting in a movable metal mold with water spraying, J. Japan Inst Metals, 50, 98-107 (1986); A. Mohan, V. Agarwala and S. Ray: Dispersion of liquid lead in molten aluminium by stirring, Z. Metallkunde, 80, 439-443 (1989); Y. C. Suh and Z. H. Lee: Nucleation of liquid Pb-phase in hypermonotectic Al—Pb melt and the segregation of Pb-droplets in melt-spun ribbon, Scripta Metall. et Materialia, 33, 1231-1237 (1995).
Berrenberg casted the monotectic alloy into a thin film at a high speed to increase the cooling rate [Th. Berrenberg: The dispersion of Pb precipitates in rapidly solidified AlPb coatings in: “Immiscible Liquid Metals and Alloys”, L. Ratke—DGM Verlag, 1993, 299-310].
Ichikawa et al. kept to mix the Al/Pb alloy even in the liquid/solid mushy zone [K. Ichikawa and S. Ishizuki: Production of leaded aluminum alloys by rheocasting, J. Japan Inst Metals, 49, 1093-1098 (1985)].
Prinz et al. casted the liquid alloy onto a moving strip or wire having a high ability to remove heat [B. Prinz and A. Romero: Process of producing monotectic alloys, U.S. Pat. No. 5,400,851 (1985)].
Bohling ensured mixing using a high-pressure melting head [P. Bohling: Verfahren zur Herstellung monotektischer Legierungen mittels statischem Mischer, German Offenlegungsschrift No. 197 12 015 (1998)].
Roósz et al. used a beam of high energy intensity for melting and intensive cooling of the substrate [A. Roósz, J. Sólyom, G. Buza and Z. Kálazi: Eljárás monotektikus ötvözetböl álló munkafelülettel ellátott fém munkadarabok elöállítására (Process for preparing metallic work-pieces provided with a work-surface of monotectic alloy), Hungarian patent No. 223,610 (2004)].
Melting and Freezing in Low-Gravity Environment
When melting and freezing are performed in a low gravity field, sedimentation does not take place, although the Marangoni force is still active. The cost of this technology is obviously very high, moreover it does not lead to perfect results due to the Marangoni convection.
Such experiments were performed by Andrews et al. in the Cu—Pb—Al system during a NASA parabola flight [J. B. Andrews, A. C. Sandler and P. A. Curreri: Influence of gravity level and interfacial energies on dispersion-forming tendencies in hypermonotectic Cu—Pb—Al alloys, Metal Trans. A, 19A, 2645-2650 (1988)] and also Liu et al. for the Fe—Sn alloy using a drop tube [X. Liu, X. Lu and B. Wei: Rapid monotectic solidification under free fall condition, Science in China Ser. E, Engineering and Materials Sciences, 47, No. 4, 1-12 (2004)].
Application of the Lorentz-Force to Prevent Sedimentation
If electric current is passed through a conductor, such as a liquid metallic alloy, in a magnetic field, then a so-called Lorentz force compensating the gravitational force acts on the droplets, and so in an ideal case a quasi gravity-free environment is created which prevents the sedimentation of the droplets.
Uffelmann et al. applied this technique for the Al—Pb system, without significant results [D. Uffelmann, L. Ratke and B. Feuerbacher: Lorentz-force stabilization of solid-liquid and liquid-liquid dispersions in: “Immiscible Liquid Metals and Alloys”, L. Ratke—DGM Verlag, pp. 251-258 (1993)]. The main reason of the failure was the appearance of the Marangoni force pulling the droplets towards the temperature gradient, which eventually lead to inhomogeneous droplet distribution and the coalescence of the droplets.
As a conclusion it can be stated that none of the methods known till now allows the production of monotectic alloys of optional thickness with a homogeneous distribution of the second phase.